There are examples (for instance, Japanese patent laid-open publication numbers: 7(1995)-6730; 8(1996)-7831; 8(1996)-31370; and 8(1996)-31372 of time-of-flight mass spectrometers that make use of the fact that a continuous flow of ions in the form of a beam (a "continuous ion beam") can be spatially separated along a flight path due to the differences in the speeds of the ions, and each ion then enters a detector at different time so that the ions to be analyzed can be separated.
FIG. 1, shows an example of such a time-of-flight mass spectrometer which contains an ion source 10 and uses a plasma to ionize a test material, a sampling cone 12 that samples atoms that have been ionized by ion source 10, a skimmer cone 14 that converts a portion of the ions that pass through sampling cone 12 into a thin ion beam, an ion lens 18 that converges the ions that pass through skimmer cone 14 and forms them into a continuous ion beam 16, a pulser 20 that provides an orthogonal acceleration to continuous ion beam 16 to cause a packet of ions 24 to flow out in an orthogonal direction, a flight tube 26 that captures ions 24 from ion output aperture 22, a deflector 28 that changes the flight direction of ions 24 that fly along flight tube 26, and a detector 30 that detects ions 24 that flow through flight tube 26.
According to FIG. 1, a sample that was ionized by ion source 10 is formed into a narrow beam by sampling cone 12 and skimmer cone 14, and thereafter is transmitted and focused into pulser 20 by ion lens 18. In the following explanation, the direction of flow of the continuous ion beam is called the "x" direction, the long direction of flight tube 26 that is orthogonal to the "x" direction is called the "y" direction, and the direction of the width of pulser 20, orthogonal to both the "x" and "y" directions, is called the "z" direction.
Pulser 20 is composed, for example, of two electrode plates 20a, 20b. One of the electrode plates (20b in the FIG.) contains an ion output aperture 22, so that ions 24 flow into flight tube 26. Ion output aperture 22 is covered with a metal mesh 22 so that the electrical field of flight tube 26 has no effect on the inside of pulser 20.
Ion beam 16 that flows within pulser 20 is a continuous beam, but, as shown in FIG. 2, when an instantaneous high voltage is applied to one of electrode plates 20, ions 24 are directed from ion output aperture 22 into flight tube 26. Because such applying time is extremely short, only the ions that are in the vicinity of ion output aperture 22 of pulser 20 are instantaneously flown out. Ions 24 that flow out in the form of packets travel along the length L of flight tube 26 and reach detector 30. As a result, the detection signal shown in the bottom portion of FIG. 2 is obtained. The flight time of an ion tm can be defined as follows in which the velocity of an ion 24 in a longitudinal direction (y direction) of flight tube 26 is v.sub.y. EQU tm=L/v.sub.y (1) EQU vy=(2eV.sub.f /m).sup.1/2 (2)
Wherein e is the ion charge, V.sub.f is a the voltage to be applied to flight tube 26 that is used to accelerate a packet of ions 24, and m is an ion mass. Apparent from equation 2, V.sub.y depends on the ion mass m, and therefore the difference in the time of flight tm can be used to perform a mass separation.
At the same time, we also have the following equation for the velocity V.sub.x of the ion in the x direction, which is orthogonal to the y direction (the x direction is the direction of continuous ion flow). Velocity V.sub.x is independent from the flight time tm but is related instead to the location at which ions are reached at detector 30. EQU V.sub.x =(2eV.sub.p /m).sup.1/2 (3)
Wherein V.sub.p is a plasma potential of continuous ion beam 16.
Velocity V.sub.x in the x direction depends on the condition of the plasma in ion source 10 and on the ion mass m, so a voltage V.sub.d is applied to deflector 28 in order to make the ions reach at the center of detector 30.
In conventional time-of-flight mass spectrometers with this construction, ion emission aperture 22 in electrode plate 20b of pulser 20 was a small hole, of about the same diameter D as the size of the aperture in detector 30 (as shown in FIG. 3), in order to create an electric field having a minimum turbulence within the pulser and thereby increase the resolution.
In a time-of-flight mass spectrometer as shown in FIG. 1, it is necessary to have a parallel beam that is narrow in the y direction within pulser 20 in order to obtain high resolution. In other words, in the y direction, which is a flight direction within the flight tube 26, high resolution requires a small dispersion, while in the direction of the continuous ion flow (x direction), as well as in the direction (z direction) of the width of pulser 20, which is orthogonal to the x direction, even if there is dispersion it has no effect on the resolution.
Therefore, in the past, as shown in FIG. 4, the final stage of ion lens 18 comprises a quadrupole lens 19 having four electrode poles 19a to generate a parallel beam that is wide in the z direction and narrow in the y direction.